1. Field of the Invention
This invention concerns a water treatment device and a water treatment method using the same, and concerns the water treatment device and the water treatment method using the same for treating water containing extremely fine objects of removal and nitrogen components.
2. Description of the Related Art
Presently, the diminishing of the amount of industrial waste, separate collection and recycling of industrial waste, and prevention of release of industrial waste are considered to be ecologically-important topics and business issues as society moves towards the 21st Century.
Some types of industrial waste comprise various types of fluids containing objects of removal; i.e., substances to be removed. Such fluids are known by a variety of expressions, such as sewage, drainage, and effluent. Fluids, such as water or chemicals, containing substances that are objects of removal, shall be hereinafter referred to as “wastewater.” The objects of removal are eliminated from wastewater by means of an expensive filtration system or a similar system. Wastewater is thereby recycled as a clean fluid, and the removed objects of removal or substances that cannot pass through the filtration system are disposed of as industrial waste. In particular, water is sent back to a natural setting, such as a river or sea, or recycled after being purified so as to meet environmental standards.
Adoption of such a filtration system is difficult because of costs incurred in constructing and running a filtration system, thus posing an environmental problem.
As can be seen from the above, wastewater treatment techniques are important in terms of recycling and prevention of environmental contamination, and immediate demand exists for a filtration system that incurs low initial and running costs.
By way of illustration, wastewater treatment as practiced in the field of semiconductors shall now be described. When a plate member formed, for example, from a metal, a semiconductor, or ceramic, is ground or abraded, an abrasion (or grinding) jig or the plate member is subject to a shower of a fluid, such as water, for preventing an increase in the temperature of the abrasion (or grinding) jig, which would otherwise be caused by friction, for improving lubricity, and for preventing adhesion of abrasion or grinding waste onto the plate member.
More specifically, in the process of dicing or back-grinding of plate-like semiconductor material; e.g., a semiconductor wafer, pure water is made to flow over the semiconductor wafer. In a dicing machine, a shower of pure water is made to flow over a semiconductor wafer, or pure water is squirted onto a dicing blade from a discharge nozzle in order to prevent an increase in the temperature of the blade or adhesion of dicing waste onto the semiconductor wafer. For the same reason, a flow of pure water is employed during an operation in which a semiconductor wafer is made thin by means of back-grinding.
Wastewater, which has mixed therein grinding or abrasion waste and is discharged from the dicing or back-grinding machine, is returned to a natural setting or recycled after having been purified through a filter. Alternatively, concentrated wastewater is recovered.
In a current process for manufacturing a semiconductor, wastewater, in which objects of removal (i.e., waste) primarily including Si are mixed, is disposed of according to either of two methods; i.e., a coagulating sedimentation method and a method, which employs a filter and a centrifugal separator in combination.
Under the coagulating sedimentation method, polyaluminum chloride (PAC) or aluminum sulfate (Al2(SO4)3) is mixed in the wastewater as a coagulant to generate a reaction product with Si and the wastewater is filtrated to remove this reaction product.
Under the method that employs a filter and a centrifugal separator in combination, the wastewater is filtrated, the concentrated wastewater is processed by the centrifugal separator to recover the silicon waste as sludge, and the clear water resulting from filtration of the wastewater is released to a natural setting or is recycled.
For example, as shown in FIG. 16, wastewater discharged during a dicing operation is collected into a raw water tank 201 and is sent by a pump 202 to a filtration unit 203. A ceramic-based or organic-based filter F is provided in filtration unit 203, and the filtrated water is delivered via a pipe 204 to a collected water tank 205 for recycling. Alternatively, the filtrated water is released to a natural setting.
In filtration unit 203, since clogging of filter F occurs, washing is carried out periodically. For example, a valve B1 connected to raw water tank 201 is closed, a valve B3 and a valve B2, for delivering washing water from the raw water tank are opened, and filter F is cleaned by a reverse flow of water from collected water tank 205. The resultant wastewater containing a high concentration of Si waste is returned to raw water tank 201. Also, the concentrated water in a concentrated water tank 206 is transported via a pump 308 to centrifugal separator 209 and is thereby separated into sludge and separated fluid. The sludge comprising Si waste is collected into a sludge recovery tank 210 and the separated fluid is collected into a separated-fluid tank 211. After further accumulation of the separated fluid, the wastewater in separated-fluid tank 211 is transported to raw water tank 201 via pump 212.
These methods have also been employed for the recovery of waste resulting from grinding or abrasion of a solid or plate-like member formed essentially from a metal material, such as Cu, Fe, Al, etc., or from grinding or abrasion of a solid or plate-like member formed from ceramic or other inorganic material.
Chemical-mechanical polishing (CMP) has come to be employed as a new semiconductor processing technology.
This CMP technique enables
(1): the realization of smooth device surface shapes; and
(2): the realization of structures with embedded materials that differ from the substrate.
With regard to (1) above, fine patterns are formed precisely using lithography techniques. The combined use of techniques for affixing Si wafers enables materialization of three-dimensional IC's.
With (2), embedded structures are made possible. Since priorly, a technique of embedding tungsten (W) has been employed in multilayer wiring of IC's. With this technique, W is embedded by a CVD method in a trench of an interlayer film and the surface is made smooth by etching back. However, smoothing by CMP has come to be employed recently. Other examples of application of this embedding technique include damascene processes and element separation.
Such CMP techniques and applications are described in detail in “Science of CMP,” published by Science Forum Co., Ltd.
A mechanism for a CMP process shall now be described briefly. As shown in FIG. 17, a semiconductor wafer 252 is placed on an abrasive cloth 251 placed over a rotary table 250, and irregularities of the wafer 252 surface are eliminated by performing lapping, polishing, and chemical etching while pouring on an abrasive (slurry) 253. Smoothing is achieved by chemical reactions induced by a solvent included in abrasive 253 and by mechanical abrasive actions of the abrasive cloth and the abrasive grains in the abrasive. Foamed polyurethane or non-woven fabric, etc., is used for example as abrasive cloth 251. The abrasive has abrasive grains of silica, alumina, etc., mixed in water containing a pH regulator and is generally referred to as slurry. Lapping is performed while pouring on this slurry 253 and applying pressure onto abrasive cloth 251 while rotating wafer 252. 254 indicates a dressing part, which maintains the abrading ability of abrasive cloth 251 and constantly keeps the surface of abrasive cloth 251 in a dressed condition. 202, 208, and 212 indicate motors and 255 to 257 indicate belts.
The above-described mechanism is arranged as a system as shown for example in FIG. 18. This system largely comprises a wafer cassette loading/unloading station 260, wafer transfer mechanism part 261, the abrasive mechanism part 262, which was described using FIG. 12, a wafer cleaning mechanism part 263, and a system controller for controlling these parts.
A cassette 264 having wafers stored therein is placed in wafer cassette loading/unloading station 260, and a wafer is taken out of cassette 264. In the wafer transfer mechanism part 261, the wafer is retained, for example, by a manipulator 265, and is placed on rotary table 250 disposed in abrasive mechanism part 262. The wafer is then smoothed by means of the CMP technique. After smoothing of the wafer has been completed, the wafer is transported by means of manipulator 266 to wafer cleaning mechanism part 263 wherein the slurry is cleaned off of the wafer. The washed wafer is then housed in wafer cassette 266.
The amount of slurry used for one abrasion process is about 500 cc to 1 liter/wafer. Also, pure water is made to flow in the above-described abrasive mechanism part 262 and wafer cleaning mechanism part 263. Since the resulting wastewater are merged in the final stage at a drain, about 5 to 10 liters/wafer of wastewater flows out during a single smoothing operation. In the case of producing, for example, a three-layer-metal wafer, about seven smoothing operations are required for smoothing the metal and interlayer dielectric films. Thus wastewater of an amount of seven times the 5 to 10 liters is discharged for production of a single wafer.
It can thus be understood that the use of a CMP machine involves discharge of a considerable amount of slurry diluted with pure water.
Furthermore, a CMP slurry, which is employed in oxide films, contains ammonia. Wastewater generated from a CMP device thus contains ammonia and other nitrogen components. In general, such nitrogen components are subject to biological treatment, which is carried out in the two steps of a nitration step of first converting ammonia nitrogen to nitrate nitrogen and a denitrification step of converting the nitrate nitrogen to nitrogen gas.
However, chemicals are used as coagulants in a coagulating sedimentation method. Specifying the amounts of chemicals that will react completely is very difficult, and hence excess amounts of chemicals are loaded and unreacted chemicals remain. Oppositely, if the amounts of chemicals are low, not all of the objects of removal will coagulate and settle out and some of the objects of removal will thus remain unseparated. Especially in a case where excess amounts of chemicals are used, chemicals will remain in a supernatant liquid, and with regard to recycling, such a supernatant liquid could not be recycled for use in applications in which chemical reactions must be avoided since the chemicals remain in the liquid even after passage through a filter.
Also, floc, which is a reaction product of a chemical and objects of removal, is generated in the form of a tuft-like suspended solid. Production of such floc is achieved under strict pH conditions and require an agitator, a pH measurement instrument, a coagulant injection apparatus, and a control equipment for controlling these components. Also, stable sedimentation of floc requires a large-size precipitation tank. For example, for a wastewater treatment capacity of 3 cubic meters(m3)/hour, a precipitation tank with a diameter of 3 meters and a depth of about 4 meters (i.e., a precipitation tank with a capacity of about 15 tons) is required. As a result, the entire system will be large-scale system requiring a floor space of about 11 meters×11 meters.
Furthermore, some of the floc float on the surface without settling to the bottom of the precipitation tank and such floc may flow out of the precipitation tank. The recovery of all of the floc is thus difficult. In short, the known filtration system suffers such problems as large facility size, high initial costs required by the system, difficulties in recycling water, and high running costs incurred by use of chemicals.
On the other hand, with a method, such as that shown in FIG. 16, which employs a filter having a filtering capacity of 5 cubic meters (m3)/hour and a centrifugal separator in combination, the recycling of water becomes possible due to the use of a filter F (which is called a UF module and comprises polysulfone fibers or a ceramic filter) in filtration unit 203. However, filtration unit 203 is equipped with four filters F and, in view of the life of the filters F, the high-priced filters F, costing about 500,000 yen each, had to be replaced at least once a year. Furthermore, since filters F are to be used with a pressure filtration method, clogging of the filters placed a large motor load and pump 202 thus had to be made high in capacity. Also, of the wastewater passing through filter F, about two-thirds are returned to raw water tank 201. Furthermore, wastewater containing objects of removal is transported by pump 202, causing the interior wall of pump 202 to be scraped by the objects of removal and thus greatly shortening the life of pump 202.
To summarize the above, the known filtration system suffers high running costs, specifically, the cost of electricity consumed by the motor and expenditures required for replacing pump P and filters F.
Furthermore, in comparison to a dicing process, an incomparable amount of wastewater is discharged during a CMP process. The slurry is distributed in the form of a colloid in a fluid and does not precipitate readily due to Brownian motion. Moreover, the abrasive grains mixed in the slurry is very minute and comprise grains with particle diameters of 10 to 200 nm. When the slurry comprising such fine abrasive grains is filtrated through a filter, the abrasive grains enter the pores of the filter and cause clogging immediately and frequently, thus making treatment of a large amount of wastewater impossible.
Furthermore, the wastewater generated from a CMP device contains ammonia and other nitrogen components and presently, such nitrogen components are removed by biological treatment. Thus two reaction tanks are required and since the treatment time is slow, the treatment efficiency is poor. Also with the biological treatment, a large-volume anaerobic tank is required to hold denitrifying bacteria and this causes the equipment construction cost to rise steeply and the device installation area to increase. Furthermore, the denitrifying bacteria are influenced significantly by the surrounding temperature environment as well as by components contained in the water to be treated and in particular, become lowered in activity in the winter when the temperature drops, thus decreasing in denitrifying action and causing the treatment efficiency to become unstable.
Furthermore, whereas the particle diameters of the abrasive grains contained in a normal CMP slurry are approximately 100 nanometers, the abrasive grains used in dry CMP, which has emerged as a new CMP method, are extremely fine particles of particle diameters of approximately 20 to 30 nanometers. Separation of such fine particles is extremely difficult.
Thus main object of this invention is to provide a water treatment device and a water treatment method using the same for treating fluids, such as CMP wastewater, that contain microparticles and nitrogen components. Another object of this invention is to provide a wastewater treatment device, which combines coagulation by electrochemical treatment and filtration treatment, and a wastewater treatment method using the same.